changes include:
1) GL_OVR_multiview to GL_OVR_multiview2 extension directive change
2) Removal of all references to side by side. We no longer support multiple views in a single 2DTexture. Only 2DTextureArray's are supported
3) WebGL 2 (ES3) is required for multiview

Bug: angleproject:3341
Change-Id: Ie0c1d21d7610f8feebdb2e4d01c6947f57e69328
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552023
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>